Ultrasonic flowmeters that work according to the transit-time principle include one or more sets of ultrasonic transducers between which a sound is transmitted. The transducers alternately act as transmitters and receivers of ultrasound. Fundamentally, a transducer consists of a piezoelectric element which transforms electric energy into sound and reversely sound into electric energy. The piezoelectric element is placed in a housing, typically stainless steel. The piezoelectric element is normally not resistant to the flowing fluid, for example district heating water. Therefore, the piezoelectric element is protected by a plate, also called a “window”, which is placed on the housing. This protecting plate or window is made of a resistant material, typically stainless steel. “Plate” hereafter means that part of the ultrasonic flowmeter through which the ultrasound is transmitted from the piezoelement to the liquid. The plate can be integrated in the measuring pipe itself, but typically the plate is part of a discrete transducer which is mounted in the measuring pipe.
The plate can be made of stainless steel comprising molybdenum which is the case for most of the ultrasonic flowmeters within the sector for energy metering, especially used for district heating measurement. Other ultrasound transducers have protection plates made of aluminium brass.
The ultrasonic transducer thus consists of two parts which are to be joined. When the two parts are made of stainless steel, they can be welded together by means of TIG welding (Tungsten Inert Gas), electron beam welding or laser welding. Alternatively the two parts can be joined together through soldering. These are the known thermal processes when the ultrasonic transducer parts are to be joined, but these processes are relatively complicated, expensive and time-consuming. Further, it is often problematic to control the temperature so that vital parts such as the piezoelectric disc are not destroyed. Cracking for example, is a well-known and widespread problem in connection with laser welding, and it is especially a problem on that type of ultrasonic transducers which have the piezolelectric disc placed in close—direct or indirect—mechanical and electrical contact with the plate. This close acoustical contact is necessary in order to transmit ultrasonic waves into the fluid.